Electrostatic coating composition comprising corrosion resistant metal particulates and method for using same

ABSTRACT

A composition comprising a corrosion resistant metal particulate component comprising aluminum-containing metal particulates, wherein the aluminum-containing metal particulates have a phosphate and/or silica-containing insulating layer; and a glass-forming binder component. Also disclosed is a method comprising the following steps: (a) providing an article comprising a metal substrate; (b) imparting to the metal substrate an electrical charge; and (c) electrostatically depositing a coating composition on the electrically charged metal substrate, wherein the coating composition comprises aluminum-containing metal particulates having a phosphate and/or silica-containing insulating layer; and glass-forming binder component.

BACKGROUND OF THE INVENTION

This invention broadly relates to an electrostatic coating compositioncomprising a corrosion resistant metal particulate component comprisingaluminum-containing metal particulates having a phosphate and/orsilica-containing insulating layer; and a glass-forming bindercomponent. This invention also broadly relates to a method for usingthis coating composition to electrostatically coat the metal substrateof an article.

A number of corrosion resistant coatings have been suggested forprotecting the metal substrate of gas turbine components, as well asother articles requiring corrosion protection. Some of these corrosionresistant coatings are applied to or deposited on the metal substrate assacrificial coatings. These coatings are “sacrificial” in that theypreferentially react with and are used up by the corrodants, thusprotecting the underlying metal substrate from attack by thesecorrodants.

Sacrificial corrosion resistant coatings for metal substrates can beformed from chromium or more typically aluminum, or from the respectiveoxides (i.e., alumina or chromia), by diffusion processes or techniquessuch as chemical vapor deposition or pack cementation. See, for example,commonly assigned U.S. Pat. No. 5,368,888 (Rigney), issued Nov. 29, 1994(aluminide diffusion coating); commonly assigned U.S. Pat. No. 6,283,715(Nagaraj et al), issued Sep. 4, 2001 (chromium diffusion coating);commonly assigned U.S. Patent Application 2004/0013802 A1 (Ackerman etal), published Jan. 22, 2004 (metal-organic chemical vapor deposition ofaluminum, silicon, tantalum, titanium or chromium oxide on turbine disksand seal elements to provide a protective coating). Diffusion coatingprocesses and techniques such chemical vapor deposition can becomplicated and expensive processes for applying sacrificial corrosionresistant coatings to the metal substrate. These diffusion coatings alsorequire some of the metal in the underlying substrate to be able todiffuse therefrom to form the coating.

These sacrificial coatings can also be applied to the metal substrate asaqueous compositions comprising phosphate binder systems andaluminum/alumina particles. See, for example, U.S. Pat. No. 4,606,967(Mosser), issued Aug. 19, 1986 (spheroidal aluminum particles); U.S.Pat. No. 4,544,408 (Mosser et al), issued Oct. 1, 1985 (dispersiblehydrated alumina particles). The phosphate-containing binder systems ofthese aqueous coating compositions typically comprise other bindermaterials, including chromates. See, for example, U.S. Pat. No.3,248,249 (Collins, Jr.), issued Apr. 26, 1966; U.S. Pat. No. 3,248,251(Allen), issued Apr. 26, 1966; U.S. Pat. No. 4,889,858 (Mosser), issuedDec. 26, 1989; U.S. Pat. No. 4,975,330 (Mosser), issued Dec. 4, 1990.

These aqueous coating compositions comprising phosphate-containingbinders are typically applied by standard “wet spray” methods commonlyused for spray painting. “Wet spray” methods are relatively easy anduncomplicated methods for applying these aqueous coating compositions tothe surface of the metal substrate of the article to be coated. However,due to the lack of precision of “wet spray” methods for applyingcoatings and to ensure adequate coverage of the surface of the metalsubstrate to be coated, a significant amount of the aqueous coatingcomposition does not end up on the metal substrate, but is instead lostdue to “overspraying” thereof. Because of environmental concerns thatcan be created by such “overspraying,” aqueous coating compositionscomprising phosphate-containing binders that are substantially free ofchromates have been developed. See, for example, U.S. Pat. No. 6,368,394(Hughes et al), issued Apr. 9, 2002 (substantially chromate freephosphate binder component). Even so, the lack of precision of “wetspray” application methods still makes it difficult to adequately anduniformly apply the coating composition to the surface of the metalsubstrate, especially a vertical extending surface, without significantand wasteful “overspraying” thereof.

Accordingly, there is still a need for sacrificial coating compositionsthat: (1) provide corrosion resistance for the underlying metalsubstrate; (2) can be applied to the metal substrate by relativelyuncomplicated and inexpensive methods; (3) can be applied so as toadequately and uniformly cover the surface of the metal substrate,especially when that surface is vertical; (4) reduce or minimizewasteful “overspraying;” and (5) do not affect the intrinsic propertiesof the base metal or have a large interaction zone at the coating-metalinterface.

BRIEF DESCRIPTION OF THE INVENTION

An embodiment of this invention broadly relates to a compositioncomprising:

-   -   a corrosion resistant metal particulate component comprising        aluminum-containing metal particulates, wherein the        aluminum-containing metal particulates have a phosphate and/or        silica-containing insulating layer; and    -   a glass-forming binder component.

Another embodiment of this invention broadly relates to a methodcomprising the following steps:

-   -   (a) providing an article comprising a metal substrate;    -   (b) imparting to the metal substrate an electrical charge; and    -   (c) electrostatically depositing a coating composition on the        electrically charged metal substrate, wherein the coating        composition comprises:        -   a corrosion resistant metal particulate component comprising            aluminum-containing metal particulates, wherein the            aluminum-containing metal particulates have a phosphate            and/or silica-containing insulating layer; and        -   a glass-forming binder component.

The composition and method of this invention provide a number ofsignificant benefits and advantages in providing sacrificial corrosionresistant coatings for articles comprising metal substrates. Thecorrosion resistant metal coating composition can be applied by anelectrostatic coating method that is relatively uncomplicated andinexpensive. The embodiments of the electrostatic coating method of thisinvention enable the corrosion resistant coating composition to beapplied adequately and uniformly on the surface of the metal substrateto provide corrosion protection. The corrosion resistant coatingcomposition of this invention can also reduce or minimize wasteful“overspraying” of the composition. The corrosion resistant coatingcomposition of this invention also does not adversely affect theintrinsic properties of the base metal of the substrate, or have a largeinteraction zone at the coating-metal interface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a turbine blade for which the coatingcomposition of this invention is useful.

FIG. 2 is a sectional view of the blade of FIG. 1 with a corrosionresistant coating formed on the blade substrate using an embodiment ofthe coating composition of this invention.

FIG. 3 is schematic illustration of an embodiment of the method of thisinvention using a tribo electrostatic spray system.

FIG. 4 is schematic illustration of an embodiment of the method of thisinvention using a corona electrostatic spray.

FIG. 5 is a sectional view similar to FIG. 2 of a corrosion resistantcoating comprising a plurality of layers formed by an embodiment of themethod of this invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “particulate” refers to a particle, powder,flake, etc., that inherently exists in a relatively small form (e.g., asize of about 150 microns or less) or can be formed by, for example,grinding, shredding, fragmenting, pulverizing or otherwise subdividing alarger form of the material into a relatively small form.

As used herein, the term “corrosion resistant metal particulatecomponent” refers to components of this invention comprisingaluminum-containing metal particulates having a phosphate and/orsilica-containing insulating layer, plus other optional corrosionresistant metal particulates. These other optional corrosion resistantmetal particulates can comprise chromium, zirconium, nickel, cobalt,iron, titanium, yttrium, magnesium, hafnium, silicon, tantalum, etc., orcombinations thereof. The particular level and amount ofaluminum-containing metal particulates and optionally other metalparticulates present in the corrosion resistant particulate componentcan be varied depending on the CTE properties desired for the resultantcorrosion resistant coating, whether the corrosion resistant coatingcomprises a single layer or a plurality of layers, etc. Typically, thecorrosion resistant metal particulate component comprises from about 1to about 40% aluminum-containing metal particulates, more typically fromabout 2 to about 20% aluminum-containing metal particulates, with thebalance being other (i.e., nonaluminum) metal particulates. Theparticulates comprising the corrosion resistant particulate componentcan have particle sizes in the range of from about 0.5 to about 150microns, more typically in the range of from about 5 to about 40microns, and can comprise particulates having unimodal, bimodal orpolymodal particle size distributions.

As used herein, the term “aluminum-containing metal particulates” refersto metal particulates comprising aluminum, or aluminum alloys, i.e.,aluminum alloyed with other metals. These other metals can includechromium, nickel, cobalt, iron, hafnium, zirconium, yttrium, tantalum,platinum, palladium, rhenium, silicon, etc., or combinations thereof.Suitable aluminum alloys include those having the formula: MAl, MCrAl,and MCrAlX, wherein M is nickel, cobalt, iron or a combination thereof,and X is hafnium, zirconium, yttrium, tantalum, platinum, palladium,rhenium, silicon, or a combination thereof.

As used herein, the term “unimodal particle size distribution” refers toa particle size distribution comprising one particle size fraction. Whengraphically plotted, a unimodal particle size distribution hasessentially a single peak.

As used herein, the term “bimodal particle size distribution” refers toa particle size distribution that comprises a smaller particle sizefraction and a larger particle size fraction. When graphically plotted,a bimodal particle size distribution has essentially two distinct peaks.

As used herein, the term “polymodal particle size distribution” refersto a particle size distribution that comprises three or more particlesize fractions. When graphically plotted, a polymodal particle sizedistribution has three or more distinct peaks.

As used herein, the term “phosphate and/or silica insulating layer”refers to an electrostatically insulating layer comprising anelectrically nonconductive phosphate, an electrically nonconductivesilica or an electrically nonconductive phosphate and silica combinationthat surrounds, encloses, encapsulates, or otherwise coats, thealuminum-containing metal particulates (as well as any other metalparticulates optionally present in the corrosion resistant metalparticulate component) so that the corrosion resistant metal particulatecomponent can be applied electrostatically to the surface of the metalsubstrate of the article. Suitable insulating layers can be derived fromtriethyl phosphate, ammonium monohydrogen phosphate, phosphorouspentoxide/phosphoric anhydride (typically dissolved in a solvent such asethanol), tetraethyl orthosilicate, etc., or combinations thereof. Theinsulating layer can have any suitable thickness such that the metalparticulates in the coating composition can be applied or deposited byelectrostatic application methods on the surface of the metal substrateof the article. Typically, the insulating layer has a thickness of fromabout 0.01 to about 2 microns, more typically from about 0.1 to about 1microns. This insulating layer can be formed on the metal particulatesby any suitable process, including vapor deposition, physical vapordeposition, chemical vapor deposition, atomic layer epitaxy, sol-gelprocessing, electroplating, electroless plating, passivation/picklingchemical treatments, aqueous or gas-phase chemical surface treatments,etc. A particularly suitable approach for forming the insulating layeron the metal particulates is by precipitation of the phosphate and/orsilica (in the form of the respective silicate precursor) from solution,for example, by a sol-gel process. When formed by a sol-gel process, thephosphate and/or silica forming the insulating layer is controllablyprecipitated from a solution containing a precursor thereof. Seecommonly assigned U.S. Pat. No. 6,379,804 (Ackerman et al), issued Apr.30, 2002 (herein incorporated by reference), which discloses a sol-gelprocess that can be adapted or modified to form the phosphate and/orsilica insulating layer on the aluminum-containing metal particulates ofthis invention.

As used herein, the term “substantially free” means the indicatedcompound, material, component, etc., is minimally present or not presentat all, e.g., at a level of about 0.5% or less, more typically at alevel of about 0.1% or less, unless otherwise specified.

As used herein, the term “glass-forming binder component” refers to acomponent that, when cured, forms a glassy matrix to which theparticulates in the particulate component are embedded in, areencapsulated in, are enclosed by, or otherwise adhered to. This bindercomponent can comprise glass particulates (e.g., glass powder, glassfrit), frits comprising other inorganic minerals, as well as phosphatebinder compounds, compositions, etc., alone or in combination with otheroptional binder materials. These phosphate binders are typically in theform of the respective phosphate compounds/compositions, includingorthophosphates, pyrophosphates, etc. These phosphatecompounds/compositions can be monobasic, dibasic, tribasic or anycombination thereof. The phosphate binders can comprise one or moremetal phosphates, including aluminum phosphates, magnesium phosphates,chromium phosphates, zinc phosphates, iron phosphates, lithiumphosphates, etc, or any combination thereof. Typically, the phosphatebinder comprises an aluminum phosphate, a magnesium phosphate, achromium phosphate or a combination thereof. Other binder materials thatcan be included with the phosphate binder include one or more chromates,molybdates, etc. See, for example, U.S. Pat. No. 3,248,249 (Collins,Jr.), issued Apr. 26, 1966; U.S. Pat. No. 3,248,251 (Allen), issued Apr.26, 1966; U.S. Pat. No. 4,889,858 (Mosser), issued Dec. 26, 1989; U.S.Pat. No. 4,975,330 (Mosser), issued Dec. 4, 1990. The binder componentcan also be substantially free of binder materials other than phosphatebinders, e.g., a substantially chromate free phosphate-containing bindercomponent. See, for example, U.S. Pat. No. 6,368,394 (Hughes et al),issued Apr. 9, 2002 (substantially chromate free phosphate bindercomponent).

As used herein, the term “corrosion resistant coating” refers tocoatings of this invention that, after curing, comprise a glassy bindermatrix having embedded therein, encapsulated therein, enclosed thereby,or otherwise adhered thereto, metal particulates from the corrosionresistant metal particulate component. Corrosion resistant coatings ofthis invention can provide resistance against corrosion caused byvarious corrodants, including metal (e.g., alkaline) sulfates, sulfites,chlorides, carbonates, oxides, and other corrodant salt depositsresulting from dirt, fly ash, concrete dust, sand, sea salt, etc.

As used herein, the term “corrosion resistant metal coating composition”refers to coating compositions comprising the corrosion resistant metalparticulate component (typically in an amount of from about 5 to about75%, more typically in an amount of from about 30 to about 60%, of thecomposition), the glass-forming binder component (typically in an amountof from about 10 to about 80%, more typically in an amount of from about30 to about 60%, of the composition), plus any other optional componentssuch as silicas, colorants or pigments, etc. The corrosion resistantmetal coating compositions of this invention are typically formulated tohave or provide a solid, flowable consistency, e.g., are formulated assolid, flowable powders, that can be deposited by electrostaticapplication methods or techniques.

As used herein, the term “curing” refers to any treatment condition orcombination of treatment conditions that causes melting of the bindercomponent to form a glassy matrix after the corrosion resistant metalcoating composition is applied electrostatically to the metal substrate,and thereby forms the corrosion resistant coating. Typically, curingoccurs by heating the applied or deposited corrosion resistant metalcoating composition at a temperature of at least about 1200° F. (649°C.), more typically at least about 1550° F. (843° C.).

As used herein, the term “article” refers to any article comprising ametal substrate and requiring corrosion protection, including a widevariety of turbine engine (e.g., gas turbine engine) parts andcomponents operated at, or exposed to, high temperatures, especiallyhigher temperatures that occur during normal engine operation. Theseturbine engine parts and components can include turbine disks andturbine shafts, turbine components comprising airfoils such as turbineblades and vanes, turbine shrouds, turbine nozzles, combustor componentssuch as liners, deflectors and their respective dome assemblies,augmentor hardware of gas turbine engines, gas turbine engine exhaustcomponents, etc. The corrosion resistant coatings of this invention areparticularly useful for articles comprising metal substrates in the formof turbine blades and vanes, and especially the airfoil portions of suchblades and vanes. However, while the following discussion of articles ofthis invention will be with reference to turbine blades and turbinevanes, and especially the airfoil portions thereof, that comprise theseblades and vanes, it should also be understood that the corrosionresistant coatings of this invention can be useful with other articlescomprising metal substrates that require corrosion protection.

As used herein, the term “CTE” refers to the coefficient of thermalexpansion of a material, and is referred to herein in units of 10⁻⁶/° F.For example, alumina which has a coefficient of thermal expansion ofabout 4 to 5×10⁻⁶/° F. at about 1200° F. (649° C.) is referred to hereinas having a CTE of about 4 to 5.

As used herein, the term “comprising” means various particulates,materials, coatings, compositions, components, layers, steps, etc., canbe conjointly employed in the present invention. Accordingly, the term“comprising” encompasses the more restrictive terms “consistingessentially of” and “consisting of.”

All amounts, parts, ratios and percentages used herein are by weightunless otherwise specified.

Compositions comprising aluminum-containing metal particulates andglass-forming binder systems, with or without additional chromatebinders or other binder materials, can be used to provide corrosionresistant coatings for various articles comprising metal substrates,such as turbine blades and vanes. It would also be desirable to be ableto apply these coating compositions comprising these aluminum-containingmetal particulates by electrostatic application methods and techniques.Because electrostatic particles are attracted to the electricallycharged metal substrate, electrostatic methods and techniques provide amore precise (targeted) application of the coating composition toadequately and uniformly cover the surface thereof so as to reduce theamount of “overspray.” Also, in the case of flowable solid (e.g.,flowable powder) coating compositions, any powder or solid “oversprayed”during electrostatic application can be relatively easily recovered andreused. Because of the electrostatic attraction of the coatingparticulates to the electrically charged metal substrate, electrostaticapplication methods can also permit the application of the coatingcomposition around corners of the metal substrate (commonly referred toas “powder wrap”).

However, it has been discovered that prior corrosion resistant coatingscomprising aluminum-containing metal particulates cannot be applied(e.g., sprayed) electrostatically so as to adhere to a metal substratethat has been electrically charged. Because prior aluminum-containingmetal particulates are electrically conductive, it has been found thatthese aluminum-containing metal particulates, when applied underelectrostatic conditions, do not retain their electrostatic charge foran adequate period of time. As a result, the aluminum-containing metalparticulates do not adhere to the electrically charged metal substrateuntil the coating composition can be cured. Instead, after the coatingcomposition is sprayed, especially on a substantially vertical surfaceof an electrically charged metal substrate, the sprayedaluminum-containing metal particulates tend to fall off.

The corrosion resistant metal coating compositions of this inventionsolve this problem by providing the aluminum-containing metalparticulates with a phosphate and/or silica-containing insulating layer.This insulating layer permits the normally electrically conductivealuminum-containing metal particulates in the coating composition to beapplied by conventional electrostatic application methods andtechniques. In particular, when the coating composition is applied,e.g., sprayed, under electrostatic conditions, the aluminum-containingmetal particulates having the phosphate and/or silica-containinginsulating layer adhere to the surface of the electrically charged metalsubstrate, even when that surface is vertically extending, for anadequate period of time for the applied coating to be cured. Indeed, thecoating composition comprising these insulated aluminum-containing metalparticulates can be applied so as to flow around corners of theelectrically charged metal substrate (i.e., “powder wrap”), and thusadhere to the corner surfaces, as well as portions of the surfacesextending beyond the corners of the metal substrate.

The various embodiments of this invention are further illustrated byreference to the drawings as described hereafter. Referring to thedrawings, FIG. 1 depicts a component article of a gas turbine enginesuch as a turbine blade or turbine vane, and in particular a turbineblade identified generally as 10. (Turbine vanes have a similarappearance with respect to the pertinent portions.) Blade 10 generallyincludes an airfoil 12 against which hot combustion gases are directedduring operation of the gas turbine engine, and whose surfaces aretherefore at risk to attack by environmental corrodants such as seasalt. Airfoil 12 has a “high-pressure side” indicated as 14 that isconcavely shaped; and a suction side indicated as 16 that is convexlyshaped and is sometimes known as the “low-pressure side” or “back side.”In operation the hot combustion gas is directed against thehigh-pressure side 14. Blade 10 is anchored to a turbine disk (notshown) with a dovetail 18 formed on the root section 20 of blade 10. Insome embodiments of blade 10, a number of internal passages extendthrough the interior of airfoil 12, ending in openings indicated as 22in the surface of airfoil 12. During operation, a flow of cooling air isdirected through the internal passages (not shown) to cool or reduce thetemperature of airfoil 12

Referring to FIG. 2, the base material of airfoil 12 of blade 10 thatserves as the metal substrate 60 having the corrosion resistant coatingof this invention can comprise any of a variety of metals, or moretypically metal alloys, including those based on nickel, cobalt and/oriron alloys. Substrate 60 typically comprises a superalloy based onnickel, cobalt and/or iron. Superalloys are generally described inKirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed., Vol. 12, pp.417-479 (1980), and Vol. 15, pp. 787-800 (1981). Illustrativenickel-based superalloys are designated by the trade names Inconel®,Nimonic®, Rene® (e.g., Rene® 88, Rene® 104 alloys), and Udimet®.

Substrate 60 more typically comprises a nickel-based alloy, andparticularly a nickel-based superalloy, that has more nickel than anyother element. The nickel-based superalloy can be strengthened by theprecipitation of gamma prime or a related phase. A nickel-basedsuperalloy for which the ceramic corrosion resistant coating of thisinvention is particularly useful is available by the trade name René 88,which has a nominal composition, by weight of 13% cobalt, 16% chromium,4% molybdenum, 3.7% titanium, 2.1% aluminum, 4% tungsten, 0.70% niobium,0.015% boron, 0.03% zirconium, and 0.03 percent carbon, with the balancenickel and minor impurities.

Prior to forming the corrosion resistant coating 64 of this invention onthe surface 62 of metal substrate 60, surface 62 is typically pretreatedmechanically, chemically or both to make the surface more receptive forcoating 64. Suitable pretreatment methods include grit blasting, with orwithout masking of surfaces that are not to be subjected to gritblasting (see U.S. Pat. No. 5,723,078 to Niagara et al, issued Mar. 3,1998, especially col. 4, lines 46-66, which is incorporated byreference), micromachining, laser etching (see U.S. Pat. No. 5,723,078to Nagaraj et al, issued Mar. 3, 1998, especially col. 4, line 67 tocol. 5, line 3 and 14-17, which is incorporated by reference), treatment(such as by photolithography) with chemical etchants such as thosecontaining hydrochloric acid, hydrofluoric acid, nitric acid, ammoniumbifluorides and mixtures thereof, (see, for example, U.S. Pat. No.5,723,078 to Nagaraj et al, issued Mar. 3, 1998, especially col. 5,lines 3-10; U.S. Pat. No. 4,563,239 to Adinolfi et al, issued Jan. 7,1986, especially col. 2, line 67 to col. 3, line 7; U.S. Pat. No.4,353,780 to Fishter et al, issued Oct. 12, 1982, especially col. 1,lines 50-58; and U.S. Pat. No. 4,411,730 to Fishter et al, issued Oct.25, 1983, especially col. 2, lines 40-51, all of which are incorporatedby reference), treatment with water under pressure (i.e., water jettreatment), with or without loading with abrasive particles, as well asvarious combinations of these methods. Typically, the surface 62 ofmetal substrate 60 is pretreated by grit blasting where surface 62 issubjected to the abrasive action of silicon carbide particles, steelparticles, alumina particles or other types of abrasive particles. Theseparticles used in grit blasting are typically alumina particles andtypically have a particle size of from about 600 to about 35 mesh (fromabout 20 to about 500 micrometers), more typically from about 400 toabout 270 mesh (from about 38 to about 53 micrometers).

The corrosion resistant coating 64 can be formed on metal substrate 60by a method comprising the following steps: (a) imparting to the metalsubstrate an electrical charge; (b) electrostatically depositing thecoating composition on the electrically charged metal substrate; and (c)curing the deposited coating composition while electrostatically chargedat a curing temperature that causes the corrosion resistant particulatecomponent (i.e., aluminum metal particulates, plus any other metalparticulates) and glass-forming binder component to form a corrosionresistant coating 64 that comprises a glassy matrix of binder to whichthe particulates in the particulate component are embedded in,encapsulated in, enclosed by, or otherwise adhered to. The coatingcomposition can be deposited on substrate 60 by any manner ofelectrostatic application, e.g., spraying, to provide an uncured layerof the composition comprising the particulates and binder. For example,the coating composition can be deposited on substrate 60 using aconventional electrostatic powder sprayer or gun system such as aNordson Sure Coat electrostatic powder spray system. See also, forexample, U.S. Pat. No. 6,796,519 (Knobbe et al), issued Sep. 28, 2004;U.S. Pat. No. 6,758,423 (Perkins et al), issued Jul. 6, 2004; U.S. Pat.No. 6,478,242 (Knobbe et al), issued Nov. 12, 2002; U.S. Pat. No.5,904,294 (Knobbe et al), issued May 18, 1999; U.S. Pat. No. 5,622,313(Lader et al), issued Apr. 22, 1997; U.S. Pat. No. 5,582,347 (Knobbe etal), issued Dec. 10, 1996; U.S. Pat. No. 5,402,940 (Haller et al),issued Apr. 4, 1995; U.S. Pat. No. 4,819,879 (Sharpless et al), issuedApr. 11, 1989; U.S. Pat. No. 4,811,898 (Murphy), issued Mar. 14, 1989;U.S. Pat. No. 4,784,331 (Sharpless et al), issued Nov. 15, 1988; U.S.Pat. No. 4,576,827 (Hastings et al), issued Mar. 18, 1986; and U.S. Pat.No. 5,582,347 (Knobbe et al), issued Dec. 10, 1996, which disclose someillustrative electrostatic powder sprayers/guns using tribo-electricand/or corona electrostatic charging systems that are suitable for useherein.

FIG. 3 schematically illustrates a tribo-electric electrostatic chargingsystem indicated generally as 100 suitable for electrostaticallyapplying the corrosion resistant metal coating composition of thisinvention. As shown in FIG. 3, system 100 comprises an air supply 120that enters fluidizing chamber 130. Chamber 130 includes fluidizing airindicated as 140, with the fluidized powder coating composition of thisinvention being indicated as 150. A porous medium indicated as 155 canbe placed between the incoming fluidized air 140 and fluidized powder150. Fluidized powder 150 enters an atomizer indicated as 160 and exitsas a mixture of powder and air indicated generally as 165. This mixtureof powder and air 165 enters a tribo-charging tube or spray gun 170.Electrostatically charged particles indicated as 180 exit the spray head190 and are attracted to the metal substrate 60, which is grounded.

FIG. 4 schematically illustrates a corona type electrostatic systemindicated generally as 200 that is also suitable for electrostaticallyapplying the corrosion resistant metal coating composition of thisinvention. As shown in FIG. 4, system 200 comprises an air supply 220that enters fluidizing chamber 230. Chamber 230 includes fluidizing airindicated as 240, with the fluidized powder coating composition of thisinvention being indicated as 250. Fluidized powder 250 enters anatomizer indicated as 260 and exits as a mixture of powder and airindicated as 265. This mixture of powder and air 265 enters a coronaspray gun indicated as 270. Enclosed in spray gun 270 is an electrode274 in contact with high voltage indicated as 278. Electrostaticallycharged particles indicated as 280 exit spray gun 270 and are attractedto the metal substrate 60, which is also grounded.

This deposited composition layer is then cured, typically be heating ata temperature of at least about 1200° F. (649° C.), more typically atleast about 1550° F. (843° C.), to form corrosion resistant coating 64.Coating 64 can be formed up to a thickness of at least about 10 mils(254 microns), and typically has a thickness in the range of from about0.5 to about 5 mils (from about 13 to about 127 microns), more typicallyfrom about 1 to about 2.5 mils (from about 25 to about 64 microns).

Coating 64 can be formed as a single layer, or can be formed as aplurality of layers. In forming a plurality of layers in coating 64,each respective layer can be formed by depositing the coatingcomposition and then curing the deposited composition, with the layersbeing built by depositing the coating composition on the underlyinglayer that was previously formed. The respective layers can have thesame or differing thicknesses. The coating composition used in formingeach of the respective layers can having the same or differing levels ofparticulate component and binder component. In addition, the level ofaluminum-containing metal particulates in the particulate component ofthe corrosion resistant coating composition can be the same or differentin the respective deposited layers of coating 64. Each layer depositedcan be cured to the same or different degrees. If desired, an outerglassy layer can be formed on coating 64 that comprises depositing andcuring a coating composition consisting essentially of the glass-formingbinder component that is substantially free of the particulatecomponent.

An embodiment of a corrosion resistant coating of this inventioncomprising a plurality of layers is shown in FIG. 5 and is indicatedgenerally as 364. As shown in FIG. 5, coating 364 comprises an innerlayer 368 that is adjacent to and overlaying metal substrate 60. Innerlayer 368 is relatively thick and typically has a thickness of fromabout 0.5 to about 5 mils (from about 13 to about 127 microns), moretypically from about 1 to about 2.5 mils (from about 25 to about 64microns). Typically, the inner layer 368 comprises from about 5 to about75% metal particulates, more typically from about 30 to about 60% metalparticulates.

Coating 364 also comprises an intermediate layer indicated generally as372 adjacent to and overlaying inner layer 368. Intermediate layer 372is typically thinner, especially relative to inner layer 368.Intermediate layer 372 typically has a thickness of from about 0.1 toabout 2.5 mils (from about 3 to about 64 microns), more typically fromabout 0.5 to about 1.5 mils (from about 13 to about 38 microns).Intermediate layer 372 can also comprise an increased or decreasedamount or level of metal particulates than that present in inner layer368. Typically, intermediate layer 372 can comprise from about 10 toabout 70% metal particulates, more typically from about 20 to about 50%metal particulates.

As shown in FIG. 5, coating 364 can further comprise an outer layerindicated generally as 376 adjacent to and overlaying intermediatelayer. (In the absence of layer 376, layer 372 would become the outerlayer of coating 364.) This outer layer 376 can comprise a metalparticulate component, but is typically substantially free of metalparticulates. Typically, outer layer 376 consists essentially of theglass-forming binder component to form relatively hard, glassy layer.Outer layer 376 is also typically the thinnest layer of coating 364,especially when substantially free of metal particulates. Typically,outer layer 376 has a thickness of from about 0.5 to about 5 mils (fromabout 13 to about 127 microns), more typically from about 1 to about 3mils (from about 25 to about 76 microns).

While the above embodiments have been described in the context ofcoating turbine blades and vanes, this invention can be used to formcorrosion resistant coatings, as described above, on the surfaces ofother turbine components or other articles. The corrosion resistantcoatings of this invention can also be formed during originalmanufacture of the component (i.e., an OEM component), after thecomponent has been in operation for a period of time, after othercoatings have been removed from the component (e.g., a repairsituation), while the component is assembled or after the component isdisassembled, etc.

While specific embodiments of this invention have been described, itwill be apparent to those skilled in the art that various modificationsthereto can be made without departing from the spirit and scope of thisinvention as defined in the appended claims.

1. A method comprising: a) electrostatically spraying a first fluidizedpowder coating composition onto an electrically charged metal substrateto provide at least a first coating layer on the metal substrate,wherein the first coating composition comprises a first level of aparticulate component and a first level of a binder component, wherein:the first particulate component comprises electrostatically insulatedaluminum-containing metal particulates being coated with a phosphateand/or silica-containing insulating layer; and the first bindercomponent comprises a glass forming binder; b) thereafter, curing thefirst coating layer to form a corrosion resistant coating on the metalsubstrate; and c) subsequent to (b), depositing and curing a coatingcomposition consisting essentially of the glass-forming binder onto themetal substrate to form an outer glassy layer.
 2. The method of claim 1wherein the metal substrate comprises a gas turbine engine component. 3.The method of claim 2 wherein the gas turbine engine component is atleast one component selected from a turbine disk, a turbine shaft, aturbine blade, and a turbine vane.
 4. The method of claim 1 wherein in(b), the curing step is carried out by heating the deposited coatingcomposition at a temperature of at least about 1200° F.
 5. The method ofclaim 1 wherein the metal substrate has a substantially vertical surfaceand wherein step (a) is carried out by electrostatically spraying thecoating composition on the substantially vertical surface.
 6. The methodaccording to claim 1 further comprising: prior to (c), electrostaticallyspraying a second fluidized powder coating composition onto theelectrically charged metal substrate to provide at least a secondcoating layer on the metal substrate, wherein the second coatingcomposition comprises a second level of the particulate component and asecond level of the binder component, and thereafter, curing the secondcoating layer.